The present invention relates to a method for establishing an electrical connection and a contact point.
A single-wire contacting method, called bonding, is known for establishing an electrical connection between at least two contact surfaces. In doing this, individual wires, in particular, gold or aluminum wires, are positioned between the contact surfaces to be bonded, using a wire-bonding tool. In bonding, the contact wire is bonded to the contact surfaces by applying ultrasonic pressure and heat. The free end of the bonding wire is first melted to form a ball, by applying thermal energy, and subsequently pressed onto the first contact surface, using a bonding capillary. The contact wire bonds to the contact surface as a result of atomic bonding forces (material fusion) arising at the boundary between the contact surface and the contact wire. During bonding to the first contact surface, the ball that was previously melted on is deformed into a nail head. The contact wire is then led to the second contact surface, using the wire-bonding tool. To prevent the contact wire from breaking away at the first contact point, the contact wire is formed into a loop. The contact wire is then pressed onto the second contact surface with the wire-bonding tool by again applying ultrasonic pressure and heat. This produces necking of the contact wire, causing the latter to form a rupture joint at which the contact wire breaks away from the second contact surface as the wire-bonding tool moves on. The contact wire is bonded to the second contact surface by a xe2x80x9cstitch,xe2x80x9d with atomic bonding forces again arising at the boundary between the contact wire and the second contact surface.
This known ball-wedge bonding method (ball bonding with the first contact surface, and stitch bonding with the second contact surface) produces a strong dependency between the materials of the contact wire and the contact surface, thus forming strong atomic bonding forces at the boundaries. Particularly when contacting the second contact surface, a relatively weak surface bonding forms between the stitch and the contact surface, resulting in contacting errors, particularly in the case of contact surfaces made of hard-to-bond materials.
The method according to the present invention offers an advantage over the related art in that it considerably improves the contact stability of the bond between the contact wire and the second contact surface. The fact that the wire-bonding tool provides the contact point with an additional contact securing element after bonding the second contact surface increases the contact stability of the second contact point (stitch or wedge) independently of the generation of atomic bonding force between the contact wire and the second contact surface.
In one preferred embodiment of the present invention, the additional contact securing element is provided by the ball shape, applied to the contact point and subsequently deformed by the bonding tool, at the end of the contact wire that remains free after contacting the second contact surface. This makes it possible, after forming the electrical connection between the contact wire and the second contact surface, to immediately form the ball on the end of the contact wire that is now free and to position it over the contact point as an additional contact securing element. A particularly preferred feature is to deform the ball with the wire-bonding tool so that the contact point overlaps, producing at least one, preferably two, additional bonding areas between the additional contact securing element and the contact surface. The atomic bonding forces generated cause the additional bonding areas to adhere to the contact surface, forming a sort of tensile strain relief for the contact wire bonded to the second contact surface. This very reliably prevents the contact wire from breaking away from the second contact surface. The possibility of the contact wire breaking away is now determined only by the rupture strength of the contact wire itself, and no longer by the adhesion between the contact wire and the second contact surface, i.e., the contact wire itself breaks before the contact point ruptures.
According to another preferred embodiment of the present invention, the production of the additional contact securing element can be precisely reproduced through wire-bonding tool settings, in particular, by programming a corresponding controller of the wire-bonding tool. This makes it possible to create identical contact securing elements among a large number of contacts, and these identical contact securing elements can be easily tested on the basis of a predictable, reproducible result. One particularly preferred feature is that a visual, preferably automatic visual, inspection of the contact point is carried out, in which the contact securing elements that are not precisely produced, i.e., according to the specified degree of reproducibility, are reliably detected. This makes it possible to achieve a sort of zero error rate in producing bonds that result in a higher production yield.
A contact point according to the present invention advantageously ensures a high contact stability between the contact wire and contact surface. Since the contact point includes an additional contact securing element which at least partially engages over the contact wire in the area of the contact point and forms at least one additional bonding surface with the contact surface, the available overall surface is advantageously increased for contacting the contact wire with the contact surface, enabling the contact point to withstand higher mechanical stresses. Particularly when used in safety-related components, this contact point can maintain highly redundant electrical connections.